How do I change a parameter value in a dashboard?

"I cant really get it to work. I have a combo box, but its is not showing the value of the parameter in the box. Just the True False in the drop down list."

If the parameter bound to your combobox has True, False in it, then True and False is  the value of the parameter when the dashboard is being loaded and the combobox is displaying those values.

When the dashboard is interacted with in a way that runs a Dashboard Extender, then you can pass in the value of the parameter that the user chose via the combobox by passing an argument to the extender rule.  If the combobox's bound parameter is called prmCboTrueFalse, then you would pass its current value to the externder rule as UserTrueFalse=|!prmCboTrueFalse!|.  Then you can extract the value in the extender rule and deal with it there.

if you want the combobox to show a previous user selected value, you have to set it by setting the value of prmCboTrueFalse via the SelectionChangedTaskResult in a Dashboard Extender rule prior to loading the dashboard.